Metal matrix composites (MMCs) are composed of reinforcing fibres embedded in a metal or metal alloy (herein collectively: “metal”) matrix. The fibres are of a material which has a higher melting point than the metal matrix.
The superior physical properties of such materials in comparison with metals make MMCs an attractive alternative to metals in applications where a combination of load-bearing strength, stiffness and lightness is required, for example in aircraft components having weight-reducing internal voids.
U.S. Pat. No. 4,625,095, the disclosure of which is incorporated herein by reference, describes a method of welding MMC articles by creating a fibre-reinforced weld comprising generally about 12 to 20 vol % of fibres. The weld metal is initially supplied via a consumable source of metal, and the fibres are supplied into the molten zone of the weld metal. In effect, the resultant weld material is itself an MMC. The method is intended to yield a weld material having properties close to those of the MMC articles being welded. The fibres are stated to preferably have a diameter in the range of from 0.1 to 0.4 microns (μm) and an aspect ratio (length/diameter) in the range of from 20:1 to 200:1.
A disadvantage with the method as described in U.S. Pat. No. 4,625,095 is that the very small particles align randomly and uncontrollably in the molten metal matrix, with the result that the physical properties of the resultant cooled solid material may suffer. A particular disadvantage that has been found is poor stiffness (Young's modulus) in one or more particular direction or at one or more particular location.
U.S. Pat. No. 4,625,095 acknowledges (passage bridging columns 2 and 3) that continuous or specifically orientated filaments may be used, to produce anisotropic properties in the composite.
However, a disadvantage with the known methods for producing anisotropic properties in MMCs is that the net shape of the MMC body typically changes during the forming process, with the result that extensive machining can be subsequently required, which is costly and inefficient.
The present invention aims to provide improved or at least alternative MMCs usable both as articles and welds.